Seat back



FIG. 1 is a left front perspective view of a seat back showing my new design and including broken lines indicating representative environmental elements;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a side elevational view, the opposite side being a mirror image thereof; and,

FIG. 4 is a rear elevational view thereof.

The views of the seat back not shown form no part of the claimed design. The broken lines depicting a seat and legs in FIG. 1 represent environmental elements only and form no part of the claimed design. 

The ornamental design for a seat back, as shown and described. 